1. Field of the Invention
The present invention relates to novel catalytic compositions for the preparation of .alpha.,.beta.-unsaturated aldehydes by oxidation of olefins in the gaseous phase.
This invention especially relates to catalytic compositions comprising a support substrate (or central core) in the form of inert solid spheres having an adherent layer of a catalytically active phase of the bismuth and iron molybdate type coated thereon, doped with potassium and phosphorus and, if necessary, with one or more other metallic or non-metallic elements.
2. Description of the Prior Art
French Patent No. 2,047,199 describes oxidation catalysts corresponding to the general formula: EQU Ni.sub.a Co.sub.b Fe.sub.c Bi.sub.d L.sub.e M.sub.h Mo.sub.f O.sub.g
in which L is particularly phosphorus, M is particularly potassium, a and b are numbers ranging from 0 to 15 with the sum (a+b) ranging from 2 to 15, c is a number ranging from 0.5 to 7, d is a number ranging from 0.to 4, e is a number ranging from 0 to 4, f has a value of 12, g is a number ranging from 35 to 85, and h is a number ranging from 0.01 to 0.5.
These catalysts are prepared by formulating a suspension in aqueous medium from various precursors of the elementary constituents of the catalysts, by adding a support (such as a silica gel) to said suspension which is essentially a paste, and by heating this to dryness to provide a cake which is then treated at elevated temperature in the presence of air and oxygen.
The catalysts are employed in the form of particles or of tablets.
These catalysts, both in bulk form and diluted, are effective, but they present difficulties over the course of an oxidation process on an industrial scale. Indeed, in a fixed bed, locally elevated temperatures may arise to initiate an undesirable violence of the reaction.
French Patent No. 2,202,729 describes that it is advantageous to employ catalysts for the oxidation of propylene to acrolein which are prepared by coating, namely, formed of a catalytically active layer of the same composition, but deposited onto the external surface of an inert support of at least 20 microns in diameter, instead of diluting it with a support introduced with the metallic salts. It is then possible to better control the evolution of the heat of reaction in fixed bed processes.
Nevertheless, this particular technique for producing the catalyst requires a significant portion thereof to be constituted by the inert support (66% by weight of the finished catalyst, according to the sole example of this '729 patent). The fraction reserved for the active phase in comparison with the former simply diluted catalysts is decreased, which results in a very disadvantageous decrease in the activity of the catalysts.
This may manifest itself industrially in the obligation either to use larger reactors to preserve the production capacity and identical operating conditions, or to increase the reaction temperature to preserve the production capacity and the size of the reactor. In the first instance, the major disadvantage is economic. In the second instance, two disadvantages are presented: the selectivity for acrolein will diminish and the activity of the catalyst will decrease more rapidly over the course of time.
U.S. Pat. No. 4,298,763 describes, for the oxidation of propylene into acrolein, a calcined catalytic composition (active phase) corresponding to the general formula: EQU Mo.sub.12 Bi.sub.0.1-4 Fe.sub.0.5-6 M2.sub.b M3.sub.c M4.sub.d M5.sub.e O.sub.x
in which M2 is nickel and/or cobalt, b is a number ranging from 2 to 12, M3 is particularly K, c is a number ranging from 0.01 to 0.1 and preferably from 0.03 to 0.09, M4 is P, d is a number ranging from 0 to 1 and preferably from 0.01 to 0.2, M5 is In and/or Na, e is a number ranging from 0 to 0.5 and preferably from 0.01 to 0.2, and x is the number of atoms of oxygen required to satisfy the valencies of the other constituents.
This active phase is deposited as a layer of thickness 150 to 1500 .mu.m onto a support which is also a central core of diameter greater than 100 .mu.m and of surface area less than 15 m.sup.2 /g.
The deposition of the layer of calcined and pulverulent catalytic material, the dimension of the particles ranging from 0.1 to 300 .mu.m, is carried out in moist medium, the support particles being vigorously stirred and controlled operating conditions moreover being required.
The layer coating the central supporting core constitutes at least 50% of the weight of the support, namely, at least 33% by weight of the finished catalyst and at most 250% of the weight of the support, namely, at most 71.4% by weight of the finished catalyst.
Prior to being used for the oxidation of olefins, the coated catalyst is dried and, if necessary, calcined at a temperature of 400.degree. to 700.degree. C.
Even if these catalysts exhibit a satisfactory catalytic activity, the very complex process for the preparation thereof militates against its reproducibility and against assuring a constant quality of the catalysts thus prepared. In addition, it is well known to this art that, when the thickness of the coating layer increases such that it constitutes on the order of 20% by weight of the catalyst, the mechanical properties of said layer are insufficient for use of the catalysts on an industrial scale in fixed bed reactors (cf., for example, U.S. Pat. No. 4,521,618).